Characterization of a cDNA encoding the 20-kDa photosystem I light-harvesting polypeptide of Chlamydomonas rinhardtii

A cDNA encoding the precursor to a major 20-kDa thylakoid polypeptide of Chlamydomonas reinhardtii (P22), previously localized to the photosystem I light-harvesting complex (LHCI), was characterized. N-terminal sequencing of P22 identified the precursor cleavage site. Genomic Southern blots and polymerase chain reaction analyses show that the gene for P22 (Lhca1*1) is single-copy and contains at least one intron. Northern-blot analyses show that Lhca1*1 mRNA is highly regulated in light-dark synchronized cells. The primary sequence and predicted topology of P22 has features characteristic of light-harvesting chlorophyll a/b-binding proteins from higher plants. Sequence comparisons indicate that P22 has significantly greater identity with the Type-I LHCI protein of tomato, compared to other LHC proteins. This result suggests that the divergence of LHCI proteins into the classes found in higher plants may have occurred early in evolution, prior to the separation of green algae and land plants.